Casting magnesium and alloys thereof



Patented Apr. 28, 1 -931 UNITED STATES rm OFFICE JOHN E. HOY, F MIDLAND,MICHIGAN, ASSIGNOR TO THE DOW CHEMICAL COMPANY, OF MIDLAND, MICHIGAN, ACORPORATION OF MICHIGAN CASTING MAGNESIUM AND ALLOYS THEREOF I NoDrawing.

lVhile the present improvements relate more particularly to the castingof magnesium and so-called light metal alloys, in which magnesium is thepredominant constituent, it will be understood that the improved methodand apparatus involved may also be found adaptable for the casting ofother similar readily oxidizable metals. The difficulties encountered inmelting and handling, in molten state, magnesium or an alloy largelycomposed of this metal are well known and these diificulties are stillpresent when it is attempted to introduce such met-a1 into a mold in anyprocess of casting. For example, when sand castings are made in theordinary way by simply pouring the molten magnesium or light alloy metalinto the sand mold not only is there a pronounced tendency for the metalto burn in the sprue, but the surface of the casting formed by contactwith the complementary surface of the mold tends to become pitted due tothe formation of a powder (presumably an oxide or a nitride of themetal) or else blistered, the formation of such blisters being somewhatobscure. Even if such impairment'of the surface is not serious enough toprevent the cast article from being properly finished when removed fromthe mold, the casting has an unsightly appearance and a certain loss ofmetal is of course involved, while the cleaner the casting, the moreeasily may the finishing operation be carried out.

It has heretofore been proposed to overcome these difliculties,specifically in casting magnesium and alloys containing the same, bydusting the mold prior tocasting the -metal with sulphur or equivalentmaterial capable of giving off in contact with the molten metal a vaporwhich is more easily oxidizable than magnesium. This procedure, however,has the objection that it is obviously diflicult correctly to gauge theamount of sulphur or equivalent material thus used, and particularly ifan excess is applied to the surface either of the mold proper or thecore, so much vapor may be generated that some of it becomes entrappedduring the period that the metal is solidifying, the re- Applicationfiled January 11, 1927. Serial No. 160,519.

sult being blow holes or surface shrinks in the casting. I I

I have now discovered that unexpectedly satisfactory results in the Wayof producing a casting with an unimpaired surface and having a brightmetallic finish, may be obtained by intermixing with the molding sand orby superficially coating such sand in the mold after the mold is formedwith substances distinctly different from sulphur both in theircharacter and apparent behavior. Specifically certain mineral acids orsalts of such acids may be thus employed with strikingly advantageousresults.

To the accomplishment of the foregoing and related ends, the invention,then, 0011- sists of the steps and means hereinafter fully described andparticularly pointed out in the claims, the following descriptionsetting forth in detail certain means and one mode of carrying out theinvention, such disclosed means and mode illustrating, however, butseveral of the various ways in which the principle of the invention maybe used.

The mold which may be formed to produce a casting of any desired'shapeor size, e. g. a piston for an internal combustion engine or likearticle, may have both parts made of sand or one part, e. g. the core,made of sand and the other part of metal, constituting a so-calledpermanent mold.

If to the sand employed in making either or both such mold partsapproximately 3 per cent. of common salt (N aCl) be intermixed, a verysatisfactory result in the way of a casting is obtained upon pouring themetal into .the mold in the usual manner; and whereas otherwise when atthe critical temperature range of molten light metal, such metal tendstofiash-oxidize, in accordance with the present invention the lightmetal may be safely brought into contact with the treated surfacesnotwithstanding such temperature. A casting with an improved surface islikewise obtained by the intermixture of sulphuric acid in approximatelythe same amount with the sand whereof the mold parts are composed. It isnot necessary, moreover, to mix the salt or sulphuric acid with thesand, but substantially equally satisfactory results are obtained byspraying the surface of the finished mold or core with aqueous solutionsof these chemicals.

It is entirely feasible Where the mold parts are treated, ashereinbefore described, to use therein ordinary green foundry sand. Theuse of addition agents such as those described in or on the sand doesnot preclude theme of sulphuror equivalent material, particularly in thesprue, if found desirable in order to prevent the molten metal fromburning.

It is somewhat diflicult to state just how these addition agentsfunction and why they give the bright metallic finish to the casting. Incontradistinction to sulphur, for example, they do not apparentlydecompose and give off a gas on being heated on coming in contact withthe incoming metal. Furthermore, they are readily soluble in water.

In addition to the particular salt and acid named above as suitable foruse' in my im proved method of treating molds for casting light metalsand alloys thereof, other alkali metal or alkaline earth metal halideshave been found suitable as well as alkali metal and alkaline earthmetal sulphates and sulphites; and for conciseness all of such agentsare designated hereinafter as oxidation-inhibitors. Furthermore, thereis no limitation implied to any particular percentage of the addedingredient employed, e. g. 3% of salt, NaCl, as hereinbefore specified.On the contrary, the amount of salt or other equivalent ingredient mayvary widely from the figure given and castings of superior I characterstill be secured.

Other modes of applying the principle of my invention may be employedinstead of the one explained, change being made as regards the means andthe steps herein dis- -closed, provided those stated by any one of thefollowing claims or their equivalents be employed.

I therefore particularly point out and distinctly claim as myinvention 1. A method of inhibiting burning of a readily oxidizablelight metal incasting, which comprises incorporating in a sand mold partan oxidation-inhibitor agent selected from the group consisting ofsulphuricacid and sulphates, sulphites, and halides of alkali andakaline earth metals, whereby such agent is in surface exposure, and isadapted to prevent burning of cast metal.

2. A method of treating a sand mold part to eliminate excessive burningof a readily oxidizable metal such as magnesium or its alloys when castin contact therewith, which comprises incorporating in surface exposurein such mold part a liquid embodying an oxidation-inhibitor agentselected from the group consisting of sulphuric acid and sulphates,sulphites, and halides of alkali and alkaline earth metals, whereby saidagent is adapted to prevent burning of the cast metal. 3. A method ofinhibiting burning of a readily oxidizable light metal in casting, whichcomprises incorporating in a sand mold part an alkali metal halide,whereby such halide is in surface exposure and is adapted to preventburning of the cast metal. 4. A method of inhibiting burning of areadily oxidizable light metal in casting, which comprises incorporatingsodium chloride in the surface of a sand mold part with which the metalwill contact, whereby to preventburningof the castmetal.

Signed by me, this 7th day of January,

JOHN E. HOY.

